Nick Clegg's latest bid to draw a line under the damaging tuition fee row has been thrown into doubt after a video lampooning his apology swept the internet.

One of the Liberal Democrat leader's most senior colleagues, Vince Cable, also risked undermining him by claiming to have been "sceptical" about the manifesto pledge to oppose fee hikes.

Mr Clegg expressed regret over the promise for the first time as the party braced for a potentially difficult conference in Brighton.

In a two-and-a-half minute film he admitted that many voters were "angry and disappointed" at the coalition's subsequent move to treble the maximum annual fee to £9,000.

He said it was a mistake, adding: "It was a pledge made with the best of intentions - but we shouldn't have made a promise we weren't absolutely sure we could deliver."

Business Secretary Mr Cable endorsed the apology and insisted he also took responsibility for the "collective" decision, but told the BBC's Newsnight programme: "I was sceptical about the pledge but we agreed collectively to do it and I take my share of the responsibility."

Mr Cable said Danny Alexander, now Chief Secretary to the Treasury, was also among those who privately warned the policy was unaffordable.

In a bizarre turn of events, Mr Clegg has also given permission for a video making fun of his apology to be released as a single on iTunes.

A version of the film created by satirical website www.thepoke.co.uk has the Deputy Prime Minister singing his remarks to a pop backing, and received thousands of views in a matter of hours. All profits will go to Sheffield Children's NHS Foundation Trust. Mr Clegg's wife Miriam is a patron of the hospital.

A source close to Mr Clegg said the video was another way of "getting the message out".

The Press Association, photo by Liberal Democrats